1. Field of the Invention
The present invention relates to a control valve for controlling an amount of air supplied to an internal combustion engine, and more particularly to a control valve for controlling an amount of air bypassing a main air passage. The bypassing air control valve is attached to a throttle body that forms the main air passage therein, and functions to maintain an idling speed of the engine at a predetermined level.
2. Description of Related Art
A process of surface-treatment of an aluminum part formed by die-casting is described in JP-A-2002-12987, for example. In this process, an anodic oxide film (anodized aluminum film) is formed on the aluminum surface before processing in solution containing bivalent metals, and then a fluororesin film is coated on the anodic oxide film.
On the other hand, an idling speed control valve for an internal combustion engine is disclosed in JP-A-2001-271727, for example. The control valve is composed of an aluminum alloy housing formed by die-casting and a valve body that rotates in the housing to change an open area of an air passage formed in the housing. Adhesive particles or materials contained in unburned gas flow back into an air passage in the housing of the control valve. To prevent adhesion of such particles or materials, the surface of the housing that slidably contacts the valve body is coated with a thin resin film made by mixing silicone and polyamideimide.
In the process disclosed in JP-A-2002-12987, the anodic oxide film can be successfully formed on a flat aluminum surface. However, it is difficult to perfectly cover the aluminum surface having defects or small voids or including an acute-angled portion with the anodic oxide film. When the fluororesin film is coated on such a surface not perfectly covered with the anodic oxide film and heated for curing (as is usually performed), air in the defects or the voids will expand and holes will be made through the fluororesin film. Thus, the fluororesin film is damaged and becomes ineffective as a film for protecting the aluminum surface.
In the control valve disclosed in JP-A-2001-271727, the sliding surface of the aluminum alloy housing is directly coated with the mixture resin of silicone and polyamideimide. However, no corrosion-resistant film is formed underneath the mixture resin film. Therefore, when the housing is exposed to strong corrosive materials (such as acid, alkaline or chlorine compounds), the corrosive materials will infiltrate into the aluminum alloy from edges or fringes of the mixture resin film, and the aluminum alloy material will be corroded.